Thus, a high velocity propulsion jet is produced. The hot gases which partially expand in the turbine are then exhausted through the discharge (propelling nozzle) by which the remaining enthalpy is con verted into kinetic energy. at low altitudes, or lower than the exit pressure, i.e. The hot gases from the com bustion chamber then pass through the turbine nozzle ring. These losses result from the fact that the atmospheric pressure will either be higher than the exit pressure of the exhaust gases, i.e. While a jet-propulsion engine creates thrust by accelerating a small amount of gas to great speeds, the exhaust of the engine contains much unused energy in the form of heat and jet engines accordingly have a low fuel efficiency. Nozzle designers typically must select a shape that is optimum at only one altitude but minimizes the losses that occur at lower or higher altitudes. The exhaust gases, which are now even more highly pressurized, then escape through the exhaust nozzle at an extremely high speed. The pressure of these gases will decrease as energy is. As we have seen previously, pressure decreases as altitude increases. The hot gases must now be expanded in the diverging section of the nozzle to obtain maximum thrust. Unfortunately, this situation can only occur at one specific atmospheric pressure on a fixed-geometry nozzle. The flow in this case is perfectly expanded inside the nozzle and maximizes thrust. The ideal nozzle is a quasi-one-dimensional nozzle flow under the following assumptions: 1.In an ideal nozzle that optimizes performance, the exit pressure (Pexit) will be equal to the ambient pressure of the external atmosphere (P¥).Of greatest concern is to design the shape and length of the nozzle so that it converts as much of that thermal energy into thrust as possible.The behavior of this expansion process is largely dictated by pressure–both the pressure of the exhaust itself as well as the pressure of the external environment into which it exhausts.
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